A mullion is a vertical member that allows two single doors to be used in a double door opening. A mullion is installed in the center of the door opening near the meeting edges of a pair of doors. The mullion is attached to the sill by a bottom fitting and to the head by a top fitting. Two strikes are mounted on opposing sides of the mullion, one for each of the two single doors.
It is often desirable to remove the mullion to provide a full double door opening. For example, if the double doors are on an auditorium, the mullion can be removed to allow large equipment to be brought into the room. Removable mullions are known in the prior art, as are removable mullions having locks to prevent unauthorized removal. For example, U.S. Pat. No. 3,000,062 to McCandless discloses a removable mullion that can be attached to a double door frame without the use of fasteners.
Often, the double doors to a large public room are also rated as fire doors. Fire doors typically have a locking mechanism between the doors with a meltable member. In the event of a fire, the meltable member in the lock is destroyed by the heat of the fire, thus keeping the doors from being opened, allowing the fire to spread. For example, U.S. Pat. No. 5,527,074 to Yeh discloses a fire protection door lock. The Yeh '074 lock includes a plastic member that restrains a spring bolt. When the plastic member melts, the spring bolt engages. Likewise, U.S. Pat. No. 5,464,259 to Cohrs et al. discloses a door latch with a meltable fuse mechanism. The Cohrs et al. '259 device is used on a door with a push pad that translates a latch assembly. When the fire fuse melts, the translation linkage is disrupted. A need exists for a similar fire safety element in the top fitting of a removable mullion assembly. Such a top fitting should allow for easy installation and removal of the mullion. It should also prevent removal of the mullion when a fire is present.